(1) Field for the Invention
The present invention relates to a method of making ram extrudable granules comprising aromatic heterocyclic polymer particles, such as polyimide particles.
(2) Description of the Prior Art
Fabrication of useful articles from aromatic heterocyclic polymers, such as polyimides, by continuous extrusion processes has proven to be difficult to achieve. In fact, aromatic heterocyclic polymers generally are intractable and are difficult to fabricate by any continuous process. Articles which can be fabricated from aromatic heterocyclic polymers by continuous processing normally are lower in density and have properties much inferior to those articles that can be obtained by compression molding the polymer.
Compression molding is usually the only method which can be used to form thick articles of the intractable aromatic heterocyclic polymers. It would be highly desirable to form such polymers into usable articles of high density by a continuous processing technique, such as the ram extrusion method which is frequently used to continuously process polytetrafluoroethylene. In this ram extrusion method, an article of high density is formed by forcing particles of a powder through a heated tube having a nonvarying cross section by successive strokes of a reciprocating ram. If this extrusion method is used to form articles from aromatic heterocyclic polymers, the rates for forming the articles are very slow and the properties of the articles formed are much inferior to those obtained on compression molding the polymers. For example, when a 10 mm diameter rod is extruded from a thermoplastic polyimide polymer, the surface of the article tends to be rough and discontinuous. The material contains large voids and significant color variations, and the flexural strength is undesirably low. In addition, the powder is difficult to feed because of the low apparent density and the need to vent or exclude a large quantity of air, and volatiles given off on heating and compacting the polymer. If the extrusion rate is increased, the noted problems generally are increased, while even at extremely low extrusion rates, the problems are not eliminated and the resulting rod is porous and is low in flexural strength.